The present invention generally relates to semiconductor integrated circuits, and more particularly to a protection circuit which protects an internal circuit formed on an LSI chip from an abnormal voltage.
It is necessary to protect internal circuits of LSI devices used in, for example, portable electronic devices from an abnormal voltage, which normally occurs abruptly. Normally, a voltage follower circuit which functions as a buffer amplifier for use in an analog signal is used in portable electronic devices. Such a voltage follower circuit includes an operational amplifier in which its inverting terminal and output terminal must be connected to a terminal in a state where they are short-circuited. The voltage follower circuit must be a low-impedance element, and thus, a low-resistance protection circuit is provided for increasing the electrostatic breakdown voltage of the voltage follower circuit.
FIG. 1 is a circuit diagram of a related analog voltage output circuit, which includes an external connection terminal 1, such as a pad, an internal amplifier 2 formed of an operational amplifier serving as a voltage follower circuit, and an output protection circuit 3. The voltage output circuit shown in FIG. 1 functions to receive an input voltage Ei from a circuit. (not shown) having a high impedance and output an output voltage Eo to a low-impedance circuit so that an amplifier gain of the circuit is equal to 1, that is, Eo=Ei. In other words, the voltage output circuit shown in FIG. 1 functions as a voltage follower circuit which converts a high input impedance into a low output impedance without changing the voltage. For this purpose, the output terminal OT and inverting input terminal IN(-) of the internal amplifier (operational amplifier) 2 are connected to the terminal 1 in the state where they are short-circuited.
Since it is necessary to realize the low output impedance of the voltage output circuit, the output protection circuit 3 is provided which protects the internal amplifier 2 from an abnormal voltage Eh applied to the terminal 1 unexpectedly. The output protection circuit 3 has two diodes Da and Db, and two resistors Ra and Rb. The resistors Ra and Rb, each having a low resistance, are connected in series between the terminal 1 and the output terminal of the internal amplifier 2. The diode Da is connected between the resistor Ra and ground GND, and the diode Db is connected between the resistor Rb and a power supply line set to a voltage Vcc higher than the ground potential.
FIG. 2 illustrates a pattern of the output protection circuit 3. The resistor Ra is formed of a P-type impurity diffusion region 4 formed in a silicon substrate 7 of, for example, N type, and the resistor Rb is formed in a P-type well formed in an N-type impurity diffusion region 5 formed in the silicon substrate 7. The diffusion regions 4 and 5 are connected to a wiring (interconnection) layer 6 via contact holes 8. The diode Da is formed at a PN junction between the impurity diffusion region 4 and the silicon substrate 7, and the diode Db is formed at a PN junction between the impurity diffusion region 5 and the P-type well in the silicon substrate 7.
The abnormal voltage Eh applied to the terminal 1 is attenuated by the resistor RA, and then discharged toward the ground GND via the diode Da. Further, the attenuated abnormal voltage Eh is attenuated by the resistor Rb, and then discharged toward the Vcc line through the diode Db. In this way, the internal amplifier circuit 2 has a high electrostatic breakdown voltage.
It should be noted that since the internal amplifier 2 must have a low output impedance, it is impossible to form each of the resistors Ra and Rb of a resistor having a large resistance higher than, for example, 100 ohms. For this reason, there is a possibility that a charge q which has not passed through the diodes Da and Db and which has not been attenuated by the resistors Ra and Rb reaches the inverting input terminal IN(-) of the internal amplifier 2. The charge q inputs the internal amplifier 2, and may destroy the PN junction of a bipolar transistor forming the inverting input terminal or a gate of a field effect transistor of the internal amplifier 2 forming the inverting input terminal. The above problem occurs more frequently as the size of semiconductor elements becomes smaller or the gate insulating film becomes thinner.